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Comment by Someone | Hacker News Reader

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http://large.stanford.edu/courses/2013/ph240/eller1/: “The power required to keep a Boeing 737-300 flying at a constant altitude and speed is 7.2 × 10⁶ watts.”

So, ballpark 7 MegaWatts, more during ascent, and add a bit if passengers want to watch movies, eat hot meals, drink soft drinks, etc.

On the plus side, if the airplane doesn’t have to carry fuel, carrying capacity could go up (with some redesign to make the plane strong enough to land fully loaded), so that 737 might be equivalent to a 747.

That 737-300 has about 125m² of wing area, and, guessing, about the same are for the body, so that would be 7MW/250m² = 28kW/m². The sun delivers about 1,400W/m², so that’s 20 times what the suns delivers on a cloudless day.

Now, most of the time, the ground station (or solar space array) beaming the energy wouldn’t be directly below/above the plane, so the required power/m² could go up a lot ⇒ I would make sure that any windows in the plane wouldn’t let the microwaves through.

Doable? Possibly, but on ground level, I guess it would be wise to use a lot more area for sending that amount of power up, to prevent killing birds.

Thanks for the figures. I'd guess that most of that power is used to combat drag, which can be lowered by going slower. The rest is probably used to combat gravity, especially during ascent, which should be improved by lowering the amount of fuel.

A small battery, and/or a ground station at the airport could provide a bit of extra power during ascent, and the battery recharged during flight. I guess the extra power shouldn't amount to a lot more than 10kW...

Depending on the wavelength, it should be fairly easy to have the plane act as a Faraday cage to shield its passengers. I guess the geometry could be slightly tweaked as well to make a larger surface area, possibly using trailing metallic wires to improve the energy collection area at a minimal cost (I get that cylindrical wires are the worst aerodynamic shape, but it should be less of a problem behind the wing).

If the wavelength is low enough, a coarse mesh could collect power quite efficiently, but you'd have to worry a lot more about radio interference.

Drag and lift are related. Wings work by literally displacing lots of air towards the ground.

If you go slower, you need bigger wings (also no one really wants longer, slower flights in cramped airplane cabins).

Unless it's cargo. Cheaper than "normal air" / faster than sea shipping is a reasonable niche.

I don't know if it's a big enough niche. There are quite a lot of costs to air cargo aside from marginal fuel efficiency and somehow developing novel cargo-specific aircraft has to translate into both increased profits for Boeing/Airbus and decreased costs to shippers. Keeping in mind that these slower craft would be unsuitable for some of the most profitable uses of air freight today. I'm not sure it makes sense.

I guess that's part of the reason we still see propeller planes around (often with the military).

Answering to a cousin comment, buses and trains can't compete when you need to cross water :)

Well, there are boats. Passenger sailing as a means of travel is not especially popular these days.

If slower meant cheaper tickets I can guarantee that people would want to use it.

The lower cost, slower mode niche would have to compete with buses and trains, with the added penalty of TSA security theater. I'm not saying you're wrong, but I'm still not sure the financials work out.

I love posts like this that remind me that I'm on HN and that HN can solve world problems based on assumptions.

Take a scenario. Give it best-case figures. See if it works. If it doesn't, then it's impossible. If it misses the mark by just a bit, it's a remote possibility. If it works, try the worst case figures and the average case ones. Of course, you can also start with the worst case ones and show that it works.

I like to approach problems this way. Plus, one can have dreams, can't they?

Once you've shown it can theoretically work, it's time to consider the side effects: best/worst. With the above, someone now controls a multi-gigawat orbiting power source.

Upsides: well, it can power useful stuff just about anywhere on (two third of) earth when there's nothing else to do. That would be really useful to compensate renewables, or other grid fluctuations. One could even imagine making airplanes slower of faster depending on demand.

Downsides: well, somebody really does control that :)

https://en.wikipedia.org/wiki/Lift-to-drag_ratio#Examples_of... says the jumbojet, at cruising altitude, has 17 times as much lift as drag.

So it seems drag isn’t taking most of that power.

Also, “I guess the extra power (for ascent) shouldn't amount to a lot more than 10kW” seems unlikely to me. It’s too low compared to the 7MW used at cruise height.

Also, if my math is correct, 1kWh lifts one kilogram about 360,000 meters, or 36,000kg (less than the weight of an empty 737) by 10 meters. So, 10kW would lift the plane by only 100 meters in an hour.

I don't know if you could kill birds with this, considering that the beam would move at something like 250 m/s during regular flight. Is microwave radiation more deadly than solar radiation?

Might be a problem at the base station, but there should be multiple of those for greater fault tolerance anyway.

I'm pretty sure the beam would be traveling at 299,704,644.54 m/s. It is electromagnetic energy.

The point where the beam is aimed would be moving with 250m/s. (or about so) Therefore the hypothetical bird which crosses the beam would only get the energy for a fraction of a second.

I don't know if this reasoning is correct or not, but definitely has nothing to do with the speed of light in air. I'm not even sure why you are bringing it up.

I think he's referring to the motion of the beam to track the target.

Sun light is about 1kW/square meter.

A 737 has a wing area of 125 square meters. To deliver 7.5MW to those wings will need 60kW/square meter.

That's a far greater energy density.

As for "time in beam" of a moving beam. Presumably one end of the beam is fixed, and depending on the target's velocity the beam might not be moving much at all (oncoming plane).

The beam’s horizontal motion speed goes down linearly with height if the beam is sent from a fixed location.

So, picking 30,000 feet for flight height and 250m/s for the plane’s speed, at 900 feet the beam’s speed would be 7½ m/s. That’s quite doable for birds.

More importantly, if the beam isn’t sent from directly below the plane, and the plane flies more or less directly towards or away from the beam’s source, the speed at which it moves horizontally goes down considerably (1/cos(α), where α is the deviation from the vertical, I think), and its width when flying through it in the direction of the beam’s source at constant height will go up considerably.

The beam’s width, and, with it, intensity, will likely be larger on the ground, but my gut feeling is this won’t be enough to correct for both these factors in all cases (corrections welcome)

I started writing a complex demonstration, but the intercept theorem, (Thales' theorem) indeeds shows quite nicely that from a ground station, tracking a plane flying at (𝒽1,𝓋1) means the beam goes at 𝓋2=𝓋1*𝒽2/𝒽1. So, 5 m/s for a bird at 200m.

That said, I was mostly imagining space-based stations.

In the above case, I talked about ground-based stations at airports. Unfortunately, those would require high angular velocities, but would already be in bird-restricted areas (airports use all sorts of tricks, including drones, sounds, people chasing them, etc, to scare away birds). You could also conceivably drastically lower the beam energy output by using multiple beams that concentrate at a single point.

Now, a fun calculation: what would be the tipping point between birds saved by not emitting carbon dioxide and birds killed by the towers, if there is one?

One last point: I'd hazard that the bird is likely to change direction if it starts running into a beam and it has time to (ie, it is not killed "instantly"). This drastically lowers the dangerous altitude range, as the bird is less likely to fly in the same path as the beam (how likely was it in the first place? One would need to go faster than the other to catch up, but not too much to actually dispense the lethal dose? I haven't done the math, but I'd say it's quite unlikely).

7MW in cruise, but keep in mind the power margins that are required for a transport category aircraft.

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